2'-Deoxy-5-(hydroxymethyl)cytidine (5-hmdC) and its base 5-hydroxymethylcytosine (5-hmC) were recently discovered as oxidation products of
2’-deoxy-5-methylcytidine (5-mdC) in mammalian DNA, particularly in stem cell DNA. 5-mdC, a C5-methylated form of
2'-deoxycytidine (dC), has been studied as an indicator of DNA methylation in epigenetics. On the other hand, the biological function of 5-hmdC is currently not clear, but it is assumed that in stem cells it might be an intermediate of an active de-methylation process. Detection of 5-hmdC in a genome has been investigated to elucidate its role.